Primary requisites for algal growth systems are photon acceptance, water, trace nutrients, and a carbon source. Carbon dioxide is a common choice for the carbon source as it is an environmentally-destructive gas (aka “greenhouse gas”) which can be extracted from the stack emissions of electrical generating facilities. With proper control of the requisite ingredients, algae can be grown and harvested continuously during sunlight hours.
There are two basic types of algal growth systems-open and closed systems. Open systems (aka “open ponds” or “open raceway” systems) consist of an enclosed pond in which the algae are fed nutrients, CO2 and are directly exposed to sunlight to permit photosynthesis. In the open raceway configuration the pond is an oval shape with a central divider and paddle wheel to induce continuous flow around this oval “race track”. U.S. Pat. No. 1,643,273 teaches the basic concept of continuous loop raceway for aquaculture.
The Department of Energy demonstrated the production of biodiesel from algae in its “Aquatic Species Program” in operation from 1979-1996. This program, while forefronting algae biofuels production, found its process non-competitive with fossil fuels, with issues of species invasion (the directed algae were quickly overcome by indigenous algae species of a lower lipid content), evaporation, and high processing costs. Open ponds have direct exposure to all environmental events. Additionally, the fixed nature of open pond design prevents change for future design enhancements and/or reconfiguration for plant layout modification. The construction of such systems typically exceeds $100/m2. On a ten year basis, the amortized yearly cost of open ponds is $10/m2, even ignoring the time value of money. Operating costs have recently been reported as low as $30/m2, yet this still renders oil cost over $10/gallon. The economics render the systems commercially impractical.
Covers have recently been added to open raceway systems, e.g. US Patent Applications Nos. 20080178739 and 2008299643. This addition lessens the environmental effects, and can reduce evaporation and improve the thermal control of the system. The cover however adds to the cost basis. And the reduced sunlight delivered to the pond surface will further erode photosynthetic performance. Yusuf Christi in “Biodiesel for microalgae” a research paper in Biotechnology Advances 25 (2007) reports findings of open ponds without covers exhibit 37% lower biomass and oil yield relative to closed systems or “photobioreactors”.
First generation closed systems or “photobioreactors” utilized transparent tubes made of rigid plastic (e.g. acrylic) through which the algal broth is pumped. The closed system provides isolation from environmental events and infiltration from other species. Greater process control is achieved, as evidenced by the higher productivity. This design is somewhat more available to design change and reconfiguration. US Patent #20090011492 teaches the use of large diameter acrylic tubes held at a highly inclined angle and having internal recirculation paths within the tubes.
While averting or reducing the drawbacks of open pond systems, the acrylic tube photobioreactors have been shown to be prohibitively expensive—characteristic costs are $190/m2, thus rendering this approach economically unsustainable. Further, research has shown that in dense broth processes (process efficiency is generally improved with higher broth density) light does not penetrate far into the broth within the tube, leaving a large dark zone.
Others have developed light-pipe systems to increase the volumetric efficiency of photobioreactors. McCall in patent applications 20080268302 and 20080220515 teach the use of parallel, edge transmitting devices mounted within the cultivation zone, to increase the depth of the photosynthetic activity. Wilson in patent application 20080160591 describes transparent panels having extended, light transmissive surfaces attached to the light impinged surface thereby extending the depth of light penetration. An alternative approach, wherein the light is gathered in solar concentrating systems and then delivered by light emitting fibers into the algae broth is described by Ono and Cuello in Design Parameters of Solar Concentrating Systems for CO2 Mitigating Algal Photobioreactor” The University of Arizona, “Energy” 29: 1651-1657. Therein the light transfer efficiency is stated to now be improved to 45%.
More recently, transparent film has been used in photobioreactors to achieve lower cost. Kerz in patent application 20080274494 teaches the construction of vertically-held sheets of plastic joined in such manner as to create horizontal flow channels which cascade downward in serial fashion, top-to-bottom as driven by gravity. Constructed in this manner, significant surface area can be developed per unit of floor area. The sheets are suspended and mechanically-rotated within a greenhouse enclosure. While this approach leverages a lower cost photobioreactor material, the added costs of the machinery and the surrounding greenhouse greatly challenge profitable operation.
Alternatively, Sears in patent application 20070048848 teaches the use of large and long transparent bags configured in dual-arrangement, having CO2 injected into the algae broth at one end connecting the two bags, and water/nutrients and harvesting occurring at the opposite connection end. Motion is imparted to the broth via a weighted roller mechanical drive over the bag, thereby squeezing the broth down the bag, in peristaltic manner. The arrangement is then similar to an open-raceway system, yet being enclosed in the bag. Therein, an elaborate containment and track support structure is displayed, impacting the design flexibility and challenging the cost model.
Cloud, in patent application 20080311649 displays a parallel arrangement of 6 inch diameter tubes made of transparent film, The separate tubes are pressured by the pumped algae broth, with no internal means of interconnection along the pathway, nor a novel means of end connection to avert substantial fitting cost. The large size of the tube induces large, unproductive dark zones.
What is lacking in current approaches is a financial-based approach to the design. The material selections, inefficient use the material in orientation and/or geometry, process equipment, and process configurations of current approaches neglect the use of profitability-driven parameters, thus precluding an economically-viable solution. Survivability through environmental events, such as a hailstorm, must also be a part of the design in order to support a viable financial model. Flexibility, that is, the ability to alter the design without incurring expenses of such magnitude as to collapse the financial model, is also of great importance, as the surrounding technology base (algae growth characteristics, carbon and nutrient sourcing, and lipid extraction) is changing due to the infancy of the industry.